1. Field of the Invention
The present invention relates to communication networks, and in particular, to communication networks in which terminals and/or users are mobile in the network. The present invention is applicable, for example, to the Internet where a terminal or user moves from one site to another. The present invention is also applicable to mobile telecommunication devices such as wireless telephones.
2. Summary of the Prior Art
Mobility management is an essential and integral part of mobile communication networks in order to ensure terminal mobility as a minimum requirement. Two distinct sets of functions need to be provided, namely location management and handover functions. Location management functions are required to keep track of mobile terminals/users as they roam and also locate them when there is a request for a connection to them. Handover functions take care of maintaining ongoing connections as users move and change their point of attachment in the network.
It is envisaged that mobile communication systems will need to possess characteristics that are far more advanced than those presently available in current systems. In effect, global and seamless roaming of users across a completely heterogeneous environment is desirable, where different network operators and service providers co-exist. These different networks may need to operate at different layers of cells and may need to use different air interfaces. Also, it is desirable that users have access to multimedia services. Furthermore, users may need to be able to roam to different networks and access the same services they are subscribed to in their home network with the same “look and feel” that they have at home.
These desirable characteristics place severe requirements on the network in terms of the mobility management protocols, bearing in mind aspects such as the amount of signalling traffic generated in the network, the available system capacity and the quality of service offered to users. The efficiency of a location management strategy is evaluated in terms of various factors namely the signalling traffic generated, the database structure it requires or how quickly connection can be set up. The optimality of a connection that is set up for communication is another factor that has to be considered. Handover is assessed in terms of how fast it can be carried out, how much traffic is lost, to what extent it affects the quality of service and also the optimality of new connections. Handover efficiency also depends on the parameters used for its initiation and control. The process of handover generates signalling traffic so that handover strategies have to address issues of how to reduce the signalling traffic and maximise the use of the capacity of the system.
Different networks have different mobility management techniques and addressing schemes. In order to do global mobility management, it is necessary to analyse aspects of routing and connection set-up, addressing, signalling and database access for different networks. In cellular systems, location management relies on a structure of databases consisting of the HLR (Home Location Register) and VLR (Visitor Location Register). Queries to the databases enable exact positioning of the mobile terminal prior to set-up of a connection. This ensures that the connection is set up along an optimal route. In data networks CDPD (Cellular Digital Packet Data), GPRS (General Packet Register Service), Mobile IP (Internet Protocol), generally routing to mobile stations is done inefficiently. Packets are always routed to the home network of the mobile terminal irrespective of whether the mobile terminal is in its home network or not. The packets are forwarded to the current location of the mobile terminal from the home network. This feature is known as triangular routing. The first objective of the research is to build an efficient mobility management strategy in data networks, specifically IP, in order to overcome the sub-optimal routing problems. This can then be extended to integrate IP with other systems. For global mobility management, there is also a need to provide a means for global addressing to accommodate the different types of addresses used by different networks. The use of the IPv6 address space for that purpose is envisaged.
The structure and distribution of the database used for mobility management has a direct impact on the signalling traffic, rate of database access and therefore the efficiency of the mobility management scheme. The optimality of the routing also depends on how the databases are used. The distribution of the database (centralised, distributed, hierarchical) is also an issue, together with the type of databases (relational, object-oriented).
Since it is envisaged that mobile users should be able to roam seamlessly and globally across different networks, they will need to have access to services transparently and should also be reachable irrespective of their location. Such expectations call for the need to develop a location management scheme that can be used across different networks in order to provide global coverage of mobile terminals. The basic requirements to achieve global roaming are that a mobile terminal has to recognisable and reachable in any network. Deriving this scheme imposes quite a challenge due to the heterogeneity of the environment given that each network has its own mobility management scheme as well as its own addressing scheme.
Location management is the process whereby mobile terminals inform the network of their whereabouts and network entities maintain updated information of location of mobiles for purposes of setting up and handing over connections. Location management can be seen as consisting of two phases: mobile tracking and mobile locating. Mobile tracking essentially involves mobile terminals notifying the network of their current location and network entities updating the information accordingly. Mobile locating is the process whereby a specific query is launched in order to determine the position of the mobile prior to setting up a connection. The location management strategy, which is dictated by the approach to mobile tracking and locating, has a direct impact on the efficiency of routing in the network, the amount of signalling traffic generated and the number of accesses to databases which are used for mobility management.
As was discussed in the report by A. Sesmun, L. Q. Liu, M. Fuente, S. Vahid and A. Munro, “Examination of mobility management techniques in current systems and outline proposals for further research”, Mobile VCE Networks Programme, Deliverable MVCE/BRS/WPN01/D12, May 1998, different networks have different mobility management schemes but in essence, two approaches can be identified depending on how the mobile locating phase is carried out. In cellular networks, mobility management relies on a structure of databases to keep track of mobile terminals as they roam. Upon a need to establish a connection to a mobile terminal, an explicit locating phase is required to determine the position of the terminal. In the Internet, the mobility management protocol Mobile IP integrates the mobile locating phase with connection set-up so that the initiator of the call immediately starts to send traffic assuming that the mobile is in its home subnet. Routers in the home network then take care of forwarding the traffic to wherever the terminal is roaming. In Wireless ATM (Asynchronous Transfer Mode), the location register solution implements an explicit mobile locating phase whereas the Mobile PNNI (Private Network to Network Interface) implementation requires that mobile locating be integrated with connection set-up. CDPD and GPRS both have recourse to an integrated approach.
The first approach is associated with the benefit that an optimal connection to a mobile can be set-up once its current location has been established at the expense of more signalling traffic being generated. The second approach ensures fast connection set-up but routing of traffic may require subsequent optimisation. Always routing via the home network is very inefficient especially if the caller and callee are effectively not far from each other.
Therefore, in the derivation of a global location management scheme for heterogeneous networks, the different aspects that need to be considered are:                (i) how to carry out mobile tracking and locating;        (ii) impact of the location management strategy on optimality of routing;        (iii) signalling traffic generated; and        (iv) the database architecture for mobility management purposes.        
Different networks also have different addressing schemes. Telephone numbers follow the E.164 specification of the International Telecommunications Union. Internet addresses, usually known as IP addresses, are of two protocol versions, IPv4 and IPv6 and are used to refer to host interfaces rather than endpoints. The Network Service Access Point format as specified by the International Standards Organisation is used for addressing in ATM networks. A global location management scheme would therefore require a uniform addressing scheme in order to recognise terminals in different networks.
In the Mobile IP protocol known as proposed by the Internet Engineering Task Force, a mobile is always identified by its home address, irrespective of its point of attachment in the network. When the mobile moves to a different subnet, or a foreign subnet, it acquires a temporary address referred to as a care-of-address. It then needs to register this address with its home agent by sending a binding update to the router. The home agent is a router on the mobile terminal's home link. Thereafter, the home agent can intercept any packets intended for the mobile and forwards them to the mobile's current location. This is the basic operation of Mobile IPv4 protocol. Mobile IPv4 leads to sub-optimal routing. All packets being routed via the home network causes a feature referred to as triangular routing, which is inefficient especially if the correspondent host and the mobile host are not far from each other.
In the protocol known as Mobile IPv6, it is proposed to optimise the routing once communication has been established between two hosts via the introduction of more signalling messages. Besides sending binding updates to the home agent, a mobile host can send similar updates to a correspondent host if it detects that it is receiving packets that are being forwarded from that host by the home agent. The correspondent host is then informed of the current point of attachment of the mobile and can send any further packets directly to the host. Although this approach subsequently optimises the routing, it does not eliminate the initial triangular routing.
Thus, in existing systems when a mobile node (terminal) moves to a foreign subnet (being a subnet other than its home subnet), it acquires a care-of-address. In Mobile IPv4, it may use a foreign agent care-of-address or a colocated care-of-address. In Mobile IPv6, it uses a colocated care-of-address only and the foreign agent is not needed. The mobile node then sends a binding update to the home agent. A binding update is an association between the permanent address and the care-of-address of the mobile. The home agent keeps a record of the binding. A resource record in the name server only stores the name of a terminal and its permanent address (and also other fields not relevant here).
When a correspondent node wants to communicate to a mobile node (or a fixed node), the correspondent node need not know if the recipient is mobile or not. The correspondent node supplies an application with the name of the host. This results in a query launched to the name server, and resolution of the name to an address yields the permanent address as a response. The correspondent node uses the permanent address to send packets to the mobile node. The home agent intercepts these packets and tunnels them to the mobile node. This routing via the home agent is sub-optimal. If the mobile node is using a co-located care-of-address, the end of the tunnel is the mobile node. If the mobile is using a foreign agent care-of-address, the end of the tunnel is the foreign agent, in which case the foreign agent extracts the packet and sends it to the mobile. Subsequent routing optimisation takes place in mobile IPv6.